The application of lighting to the automotive industry is well known. The original electric light sources were filament bulbs which offered high luminance from a small source. Improvements in light source design led to halogen type filament bulbs, high intensity discharge (HID) bulbs or high brightness light emitting diodes (LED). These offer improvement in terms of luminance and energy use over preceding filament bulbs. In order to apply these light sources to automotive front lighting and realise the beam spot distributions required by regulatory bodies, such as the United Nations Economic Commission for Europe (UNECE) or Federal Motor Vehicle Safety Standards (FMVSS), for the U.S.A, modification of the output beam to form specific beam spot distributions on the road is necessary. For projector headlights this requires removal of a portion of the light from the projected beam which ultimately forms the beam spot, to create a dipped beam. The dipped beam is necessary to avoid causing glare to oncoming road users. By necessity, the dipped beam also creates a restricted view of the road due to restricted illumination of the same. The removal of light is performed by a shield, which is inserted into the light path thereby causing a reduction in optical efficiency of the projector headlight.
The filament and discharge light sources provide no means for modification of the output from the source. Therefore, a shield is the only method of providing the dipped beam spot distribution pattern. To switch between a dipped beam and a driving beam, the beam pattern that is necessary for better visibility, either two headlights must be provided, one to create the dipped beam and the other to create the driving beam, or a mechanical switching mechanism must be provided. When the driving beam is desired, the mechanical switching mechanism removes the shield from the projected beam profile allowing all light to exit the projector headlight unit unimpeded.
The provision of only a dipped beam distribution, or of only a driving beam distribution, has limitations in terms of road user safety by not providing simultaneous optimal illumination of the road and minimal glare to other road users. This can be improved upon by the addition of an adaptive element to the projected headlight beam. However, all methods of creating an adaptive beam spot from a single projector unit require mechanical moving components within the headlight unit. This has a limitation on cost reduction and reliability of the headlight over the course of its lifetime. Alternative methods of provision of an adaptive beam spot require multiple light source units, which increases the headlight cost, and which also have a large volume, this having implications for pedestrian safety in the event of a collision.
Laser based light sources offer advantage over existing light sources due to the ability to control the emission from the laser diode effectively using optics with a much reduced size, and therefore, weight. This control ability stems from the small emission area and restricted angular distribution of the laser diode. The light emitted from laser diodes is often illuminated onto a fluorescent material to convert from the first wavelength to a second wavelength, which is predominantly white. The light source created is very small and can be used more efficiently with headlight projection optics.
The following background art describes the use of lasers in automotive headlight units:
US 2011-0194302 A1 (Sharp, 11 Aug. 2011); an illustration of this patent is shown in FIG. 1. This patent discloses a rectangular parallelepiped light emitting element 11 which is formed from a fluorescent material. The light emitting element 11 is excited by light from multiple laser chips 12 in a group 15. The light is concentrated to the light emitting element 11 by light guide irradiation members 13. The excitation light illuminates the light emitting element 11 via an array-like arrangement of the light guide irradiation members 13. The light emitted from the light emitting element 11 is collimated by a reflecting mirror 14 to form a headlight beam spot.
JP 2010-232044A (Stanley Electric, published 14 Oct. 2010); an illustration of this patent is shown in FIG. 2. This patent discloses a lamp for a vehicle 21 consisting of a plurality of light emitting diode (LED) light sources 22, a phosphor 23 and a shade 24. The phosphor 23 emits white light in response to excitation from a laser light source. The light distribution created by the combination of the LED 22, phosphor 23 and shade 24 is imaged through a convex lens 25 to form a dipped beam spot distribution on the road. The invention provides for a dipped beam spot distribution only from this light source.
JP 2011-134619 A (Stanley Electric, published 7 Jul. 2011); an illustration of this patent is shown in FIG. 3. This patent discloses the use of a solid state light source 31 which can be scanned across a fluorescent material layer 32 to form a controllable light source, emitting white or coloured light. The scanning is performed by a moveable reflective method 33, for example mirror. The light is then projected by means of an optical system 34, for example a lens. A similar configuration of light source is disclosed in EP 2,063,170 (Audi AG, 27 May 2009)
U.S. Pat. No. 6,736,524B2 (Valeo Vision, published as US 2003/0223246 on 4 Dec. 2003); an illustration of this patent is shown in FIG. 4. This patent discloses a projector headlight unit which is designed to emit a dipped beam spot profile onto the road. A secondary light source 41, comprising an optical distribution element 42, is incorporated into the headlight at the focal point of the principal lens 43. A laser diode 44 emits infra-red light, which is distributed by an optical distribution element 42 to produce a secondary light beam. The optical distribution element 42 is shaped in a similar fashion to the dipped beam profile by its application to the obscuring part 45. The obscuring part 45 serves the same function as the previously described shield. The secondary, infra-red light beam may be used for illuminating axially in front of the projected dipped beam.
U.S. Pat. No. 7,654,712 B2 (Koito Manufacturing, published as US 2008/0013329 on 17 Jan. 2008); an illustration of this patent is shown in FIG. 5. This patent discloses a lamp module 51 formed from multiple elements formed from a fluorescent substance 52 excited by individual light emission parts 53 to emit white light. The fluorescent emission from the fluorescent substance 52 is collimated by a plurality of micro lenses 54. This light can then be projected by another lens to form a beam spot. This has the capability to form an adaptive beam spot.
WO 2009/131126 (Koito Manufacturing Co., Ltd, 29 Oct. 2009) proposes a “lighting fixture” for a vehicle, in which a semiconductor light-emitting element illuminates a fluorescent material via a wavelength-selecting filter. The wavelength-selecting filter transmits light from the light-emitting element, but reflects light of the wavelengths re-emitted by the fluorescent material. The light from the fluorescent material, and any unabsorbed component of the original radiation from the light-emitting element, are then emitted. In one embodiment three separate light sources are provided to provide increased output intensity, but these light sources cannot be operated independently from one another. Moreover, each light source illuminates the same region of the phosphor and so each light source provides the same illumination pattern as one another. Furthermore, the phosphor surface is collinear with the optical axis, resulting in poor reproduction of any distribution upon the phosphor in the far field.
JP 2011-204406 (Caravell Co Ltd, 13 Oct. 2011) proposes an illumination device for a motor vehicle, for example for a taillight. An LED illuminates a “fluorescent diffuser”—for example the LED may emit blue light, and the fluorescent diffuser may convert this into red light. It is however not possible to vary the output illumination pattern.
U.S. Pat. No. 4,722,037 (Davis, 26 Jan. 1988) is directed to the use of a holographic optical element that provides different diffraction effects on light from two different light sources of different spectral characteristics. Two separate light sources with different spectral characteristics are provided for illuminating an holographic element—and different output patterns can be obtained depending on which of the light sources is illuminated.